What is the hardest thing to learn about English?

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In your experience of learning English, what did/do you find most difficult? Is it phrasal verbs? The various question forms (embedded questions, question tags, subject/object, and so on)? Singular adjectives? Modal verbs? Irregular verbs? compound nouns? Weird, highly erratic spelling-to-pronunciation "rules" and "exceptions"?

What is the toughest part of learning English, which made/makes the language really hard work for you?